Recently, virtual image display devices displaying an image, formed by a small-size display element, in an enlarged state as a virtual image have been progressively developed as head mounted displays or head-up displays. Such a virtual image display device is configured to project light, output by the display element, toward the eye of an viewer by use of a light guide plate, a combiner or the like. A see-through type virtual image display device is capable of displaying a virtual image of the image formed by the display element in a state of overlapping an outside view seen through the light guide plate or the combiner. Use of such a virtual image display device allows an AR (augmented reality) environment to be provided easily.
Patent Document 1 discloses a see-through type virtual image display device displaying a virtual image in a state of overlapping as outside view. This virtual image display device includes an image optical system including a display element, and an optical guide member. Light from the display element is incident on the optical guide member (light guide plate) via a coupling portion located at an end thereof as being collimated. The collimated light incident on the coupling portion is propagated inside the optical guide member, then is reflected by diffraction grating for image extraction, and is output from an light output surface of the light guide member. The diffraction grating for image extraction includes a concaved and convexed inclining surface having a sawtooth-like cross-section and a half-reflective film provided on the inclining surface. The viewer can see the virtual image deep inside the light guide member via the half-reflective film.
Patent Document 2 also discloses a see-through type virtual image display device. The light guide plate used in this virtual image display device includes a substrate having a sawtooth-like surface having a plurality of concaved portions and a relative thin substrate having a surface having a plurality of convexed portions corresponding to the concaved portions. The two substrates are bonded together. At a face where the concaved portions and the convexed portions face each other, a reflective film is provided. In this structure, the reflective film is located in the vicinity of a surface of the light guide plate, so that light propagated inside the light guide plate is reflected toward the viewer.
Patent Document 3 also discloses a virtual image display device including a reflective array reflecting light propagated inside the light guide plate to output the light outside. The reflective array described in Patent Document 3 includes a reflective surface inclining with respect to a light output surface of the light guide plate, a parallel surface parallel to the light output surface, and another inclining surface provided between the reflective surface and the parallel surface.